The hypothesis for this project is that a novel compound, Peptide Inhibitor of complement C1 (PIC1), will inhibit antibody-initiated, complement-mediated cell destruction in a xenotransfusion animal model. Complement is the most potent inflammatory cascade in humans and is initiated by antibodies driving classical pathway activation and host cell destruction in many inflammatory diseases. One such disease is Acute Intravascular Hemolytic Transfusion Reaction (AIHTR), which is a highly lethal form of transfusion reaction. Frequently transfused populations (e.g., sickle cell disease) are at greatly increased risk of AIHTR, due to development of antibodies against minor erythrocyte antigens. Currently, no treatment exists to prevent or treat AIHTR besides supportive care and thus, represents a critical unmet medical need including underserved populations like sickle cell disease patients. We have developed a simple and robust model of AIHTR in rats, which we will use to test PIC1. Our current lead compound of PIC1 (PA-CPEG), is the product of years of rational drug design yielding a 15 amino acid peptide conjugated with PEG. Our compound binds to the initiating component of the classical pathway, C1, efficiently blocking antibody-initiated complement activation at the first step in the cascade. PIC1 is delivered in a saline vehicle and has reproducibly shown in multiple dosing studies to block classical complement activation in rats. This inhibition is rapid, 30 seconds, and potent, >90% inhibition of classical complement activation. We have performed a pilot experiment using PIC1 to inhibit hemolysis of mismatched erythrocytes in our xenotransfusion protocol. PIC1 demonstrated a dose- dependent effect and at high dose profoundly inhibited complement-mediated intravascular hemolysis to the same degree as the gold-standard cobra venom factor (CVF). Thus, this pilot experiment demonstrated that PIC1 can completely block antibody-initiated classical complement-mediated cell destruction. The proposed studies will refine the dosing of PIC1 and then conduct fully powered studies to evaluate the efficacy of PIC1 in inhibiting AIHTR in the rat model. We will test PIC1 in both a prophylactic strategy and an intervention treatment strategy. The prophylactic treatment strategy will mimic the clinical scenario of the patient at high risk fo AIHTR who could potentially receive PIC1 prior to transfusion. The intervention treatment strategy will mimic the clinical scenario where the transfusion begins and the patient develops fever, diaphoresis and hypotension. In addition to receiving epinephrine to support blood pressure, the patient could also receive PIC1 to inhibit the mechanism of pathogenesis and prevent further hemolysis and acute kidney injury. The success of the proposed studies will provide critical proof-of-concept that PIC1 can prevent complement- mediated pathogenesis in an animal model of human disease. This will provide the necessary evidence to propel the future pre-clinical development of PIC1 through pharmacokinetic and toxicology studies.